1. Field of the Invention
The present invention relates generally to fuel assemblies for a nuclear reactor and, more particularly, is concerned with improved hold-down spring clamps on the top nozzle of the fuel assembly and a method of installing the hold-down springs.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle.
In the conventional fuel assembly, coolant flowing upward past the fuel rods and guide thimbles induces significant upward forces. These forces are countered by a combination of the weight of the fuel assembly and a plurality of hold-down spring assemblies of a hold-down device which push against the upper core plate of the reactor. The hold-down spring assemblies thereby prevent the force of the upward coolant flow from lifting the fuel assembly into damaging contact with the upper core plate, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like.
Heretofore, the hold-down spring assemblies of the hold-down device have been fastened and held in their operative positions on the top nozzle by using several clamps in the form of corner blocks separate from the top nozzle and a number of screws which extend through the corner blocks and bases of the spring assemblies and together with the corner blocks clamp the spring assemblies at their respective bases to a peripheral top flange of the top nozzle. In their operative positions, the spring assemblies extend in inclined upward relationships along and within the outer perimeter of the top nozzle enclosure where they contact the upper core plate.
More particularly, each hold-down spring assembly of the hold-down device is attached to the fuel assembly at the top nozzle by a single spring screw. The screws are located such that for four hold-down spring assemblies, two pairs of spring screws are located at the opposite corners of the top nozzle. The corner clamp block with recessed holes defined in it sits over each pair of spring screws and acts as a protective cover for the heads of the spring screws. As shown in U.S. Pat. No. 3,791,466 to Patterson et al. and assigned to the assignee of the present invention, each corner clamp block is attached to the top nozzle by welding and/or a single clamp screw. As shown in pending U.S. patent application entitled "Method of Removing and Replacing Locking Screws in a Nuclear Fuel Assembly" by John M. Shallenberger et al., filed July 29, 1987, Ser. No. 079,208 (W.E. 52,786I) and assigned to the assignee of the present invention, each spring screw is locked against rotation and is retained in place by a lock pin which is welded to the inside of the screw head clearance hole in the clamp block.
There have been a few known occasions when the head of a spring screw has broken off. Problems can result on such occasions when both screws fail at a single clamp location. The leverage forces exerted by the now unclamped hold-down spring assemblies can cause the clamp block-to-nozzle welds to fail. The result of such a failure is that both the clamp block and spring screw heads are now unrestrained and free to move in the coolant flow and the spring assemblies free to pivot about relative to the top nozzle. Such unrestrained spring assemblies are likely to interfere with the operation of the control rod assemblies.
Consequently, a need exists for improvements in the manner in which the hold-down spring assemblies are retained on the top nozzle.